The present invention relates to a color image forming apparatus such as a plain paper copy machine (which will be abbreviated as PPC—Plain Paper Copy machine—hereinafter) or a printer which prints and records on an image forming medium such as plain paper or a transparency (such as an OHP—OverHead Projector—) sheet image data supplied from an image information supplying apparatus such as a personal computer (which will be abbreviated as PC—Personal Computer—hereinafter) or a digital camera or image data read by an image input apparatus. More particularly, the present invention relates to a color image forming apparatus which adds a specific hidden pattern when a color image is formed on the image forming medium.
Here, the specific hidden pattern means a pattern inherent to a color image forming apparatus, with which the color image forming apparatus which has printed using a specific color that is hard to be identified by the naked eye, for example, a color material of yellow can be specified with respect to not only an image that must not be reproduced but all the reproduced images in order to specify a color image reproducing apparatus from the reproduced images printed on a medium even if a specific original which must not be essentially printed (reproduced), e.g., bank bills or marketable securities is printed.
Conventionally, in the image forming apparatus for adding a specific hidden pattern, a hidden pattern must be added in case of inputting a usual original to output an image. In case of outputting a test pattern in a test mode and the like used for, e.g., adjusting the image forming apparatus, however, a problem occurs during adjustment if a hidden pattern using a color material of, e.g., yellow is added. Adding no specific hidden pattern may be convenient in terms of adjustment in some cases. Therefore, when a test pattern is outputted from the image forming apparatus, control is carried out so as not to add a specific hidden pattern.
FIG. 1 is a block diagram showing a structure of a conventional hidden pattern addition mechanism in a color image forming apparatus. In FIG. 1, the conventional hidden pattern addition mechanism is constituted by a specific pattern generation portion 1 for generating a specific pattern; a first register 2 for generating a signal “0x00” (hexadecimal) as an addition prohibition signal directing to inhibit printing a specific hidden pattern on an output image; a second register 3 for generating by software a specific pattern addition signal as a control signal for adding a specific pattern; a selector 4 for respectively inputting the specific pattern signal as an input X, the signal “0x00” as an input Y and the specific pattern addition signal as a selective control input S to supply a specific hidden pattern from an output Q; and an adder 5 for receiving yellow image data as an input A and the output Q of the selector 4 as an input B.
In the conventional hidden pattern addition mechanism shown in FIG. 1, the specific pattern generation portion 1 generates a specific hidden pattern; the register 2 generates by software the signal “0x00” as the addition prohibition signal directing to prohibit printing of a specific hidden pattern on an output image; and the register 3 generates by software the specific pattern addition signal as a control signal. These signals are respectively supplied to the selector 4. The selector 4 selects a signal level of the inputs X and Y based on a signal level of the selected signal and outputs the selected signal to the adder 5. The adder 5 adds, e.g., a yellow signal of the inputted image data as the input A and the output of the selector as the input B to output “A+B” to a printer engine. If a specific pattern for printing a specific hidden pattern is included in the input B, the hidden pattern is printed on the image forming medium at a predetermined position.
In this manner, in the conventional mode by which a hidden pattern is not added only when a test pattern is outputted from an apparatus for adding a specific hidden pattern, a control signal which is set by, e.g., the second register 3 and generated by software is used as a control signal indicating whether a specific hidden pattern is added.
With such a control signal generated by software, however, a problem is necessarily generated even if verification of the software is performed in detail. A specific hidden pattern cannot be added due to this problem when usual printing is made, and an image alone may be possibly printed and outputted.
In such a situation, the image forming operation is an erroneous operation. Further, when a color image forming apparatus is used for precise forgery of bank bills or marketable securities, the image forming medium on which an image having no hidden pattern added thereto is formed getting into circulation is not preferable in the social system and the legal system despite the fact that addition of a hidden pattern is obligate in order to specify an apparatus.